Certain embodiments of the present invention relate to contrast imaging in ultrasound. More particularly, certain embodiments relate to enhancing contrast-to-tissue ratio (CTR) and signal-to-noise ratio (SNR) in ultrasound contrast imaging.
Contrast agents for ultrasound are being developed by several pharmaceutical companies (see U.S. Pat. Nos. 5,410,516 and 5,678,553). When injected into blood, these 1-10 um microbubbles increase the ultrasound echo strength from blood flow and perfusion (the circulation of blood to an organ or tissue). In the presence of tissue, the tissue echo strength must be significantly reduced relative to that of the contrast agent in order for flow or perfusion to be visualized. One way to suppress the tissue signal is to image the nonlinear signals generated by the microbubbles, e.g. harmonic signals, which are stronger than those generated by tissue.
Basic harmonic imaging transmits a narrowband signal at f0 and then images the harmonic signal generated by contrast agents (and tissue) at 2 f0 by bandpass filtering (see U.S. Pat. Nos. 5,724,976 and 5,733,527). Alternatively, pulse inversion permits overlap of the fundamental and harmonic bands for better resolution by using two phase-inverted transmit pulses to cancel the fundamental (linear) component which leaves the nonlinear components to be imaged (see U.S. Pat. Nos. 5,632,277, 5,706,819, and 6,371,914).
U.S. Pat. No. 5,632,277 describes a phase inversion method using amplitude modulation. In the pulse inversion method described in U.S. Pat. No. 5,706,819, the transmitting frequency band is separated from the receiving frequency band and the harmonic component in the incident pressure wave must be suppressed to detect the harmonic response of microbubbles.
Another way to enhance the bubble signal is to excite the bubbles before imaging the bubbles. This is done by transmitting an excitation pulse separated from the imaging pulse to expand the size of the bubble to get bigger scatter cross section U.S. Pat. No. 5,833,615). U.S. Pat. No. 5,833,615 states that an excitation signal separated from the imaging signal must be transmitted before the imaging signal. As a result, either an extra transducer is needed or two separate pulses must be transmitted, reducing frame rate. Also, since the bubble signal is enhanced by exploiting the linear property of the bubble, when the method is applied to low mechanical index (MI) phase inversion imaging, the enhanced signal will be cancelled and no benefits are achieved.
Also in U.S. Pat. No. 5,833,615, an excitation enhanced ultrasound system is presented. An excitation pulse separated from the imaging pulse is used in that system to excite and thus expand the size of the microbubbles that are below 4.2 um which will increase the amount of the returned ultrasound imaging signals scattered and returned by an object (contrast bubbles). This is based on the linear property of the micro-bubbles. Thus, when applied in pulse inversion mode, the increase of the returned ultrasound imaging signals may be cancelled since it is mainly based on the increase of the scatter cross section of the bubbles which will be the same for the two phase inverted pulses.
U.S. Pat. No. 6,371,914 B1 discloses a single waveform pulse inversion method. A double-pulse excitation waveform is transmitted into the media. Normally, the two pulses in the waveform are phase-inverted. In receiving, different deconvolution functions are employed to take out the echoes from the first pulse and the second pulse, then the two echoes are realigned in time and summed to cancel a linear response. U.S. Pat. No. 6,371,914 B1 states that the waveform could be considered as a convolution of any single pulse with a known coding function.
For each of the conventional methods discussed above, the ratio of contrast to tissue signal strength is still insufficient for imaging perfusion. One method to improve the contrast-to-tissue ratio (CTR) is to reduce the transmit mechanical index since the tissue nonlinear signal falls faster than that for contrast as the mechanical index (MI) decreases. But the method quickly runs into signal-to-noise ratio (SNR) limitations.
In U.S. Pat. No. 6,213,947 B1, coded transmit pulses (chirp) are also used for single firing harmonic imaging and pulse inversion harmonic imaging modes. However, in the claims, “at least some of the received signals” need to be applied to “at least one compression filter” in single firing situation or before the combination of the received signals from the first and second transmission in the pulse inversion situation. This is significant since a compression filter is larger than a regular filter and much more expensive.
In U.S. pat. Pub. appl. No. 2002/0128555 A1, two waveform components, one selected for fundamental tissue imaging, and another one selected for tissue harmonic imaging are included in one transmit signal to simultaneously perform ultrasonic fundamental and harmonic tissue imaging. Although the inventors mentioned that this method could be used for contrast imaging, no benefit can be seen for tissue cancellation and bubble response enhancement. This method is designed for tissue harmonic imaging. For example, in the patent body, in two-transmission mode, the two transmitted bursts are only partially (one component) phase inverted; the harmonic generating component is inverted but not the fundamental component. The result is that the tissue signal is not optimally cancelled. Phase inverting both components would not provide an improvement in CTR or SNR relative to conventional wideband pulse inversion methods.
A basic need still exists to increase the CTR and SNR in ultrasound contrast imaging without extra and expensive devices. Also, low-MI imaging is desired to reduce contrast agent destruction. Contrast microbubbles are destroyed by high-MI ultrasound pulses, therefore, low-MI pulses are desired in order to not destroy contrast agents and in order to maintain a longer duration over which the contrast agents may be imaged.